As is well known, the human visual system is trichromatic. All colors can be matched by a mixture of three primary colors, namely red, green and blue. This trichromatic property of human color vision is a consequence of three different photoreceptor types in the retina: the R receptor (maximally sensitive to the red end of the spectrum), the G receptor (maximally sensitive to the middle, or green part of the spectrum), and the B receptor (maximally sensitive to the blue end of the spectrum). The ratios of red, green and blue primaries in a color mixture that is made to match some arbitrary color are determined by the spectral absorption of the characteristics of the photo pigments in the three types of receptors.
Approximately 8% of males and 0.2% of females have abnormal color vision due to a defect inherent in the X-chromosome. One form of color vision defect, called anomalous trichromacy, is characterized by abnormal spectral absorption characteristics of the R receptor photopigment (protanomalous trichromat) or of the G receptor photopigment (deuteranomalous trichromat). Anomalous trichromats still require three primary colors in a mixture to perform color matching. However, the ratio of the primaries is anomalous in comparison to the normal population. A second form of color vision defect, called dichromacy, is characterized by an absence of the R receptor photopigment (protanope) or an absence of the G receptor photopigment (deuteranope). Protanope and deuteranope require only two primary colors and a mixture to perform color matching.
X-linked color vision defects are diagnosed on the basis of tests that are standardized against color matching. In particular, a yellow lamp is matched by normal trichromats with a unique mixture of a red primary and a green primary. Relative to the normal match, protanomalous trichromats add too much red to the mixture and deuteranomalous trichromats add too much green to the mixture. Protanope and deuteranope match the red alone to the yellow, green alone to the yellow, and any ratio of red to green in a mixture to the yellow. The major difference between protanope and deuteranope is in the sensitivity thereof to colored lamps. Protanopes are very insensitive to red light, consequently red lamps appear much dimmer than do yellow or green lamps that look to be the same brightness for normals. Deuteranopes have the same sensitivity to colors as does the normal.
In addition to inherited red-green color vision deficiencies, some people suffer from losses in the ability to discriminate blue from yellow. These losses are most frequently caused by disease processes such as diabetes mellitus or glaucoma, but may also be caused by the normal yellowing of the lens of the eye with age. Blue-yellow defects are also diagnosed on the basis of tests that are standardized against color matching. In these tests, there is somewhat more variety than in the red-green tests. The most common tests are: a mixture of blue-violet and green are matched against a mixture of blue and yellow lights (the Moreland equation), a mixture of blue and yellow are matched against a white light (the Pickford-Lakowski equation), or a mixture of blue-violet and green are matched against a blue-green light (the Engelking-Trendelenberg equation).
A variety of methods and instruments are used to test the above discussed color vision deficiencies of a patient. One of the more common methods involves the use of pseudoisochromatic plates. These plates comprise a series of dots of different hues of color printed on a heavy white paper board. By mixing certain colors, specific dots can be varied in color to form patterns which can be, for example, in the form of a numeral or a letter. A normal patient shown a particular card and asked to identify the pattern of the different colored dots could readily do so whereas a patient with a color deficiency would identify all dots as being the same color thereby being unable to recognize the numeral or letter.
Pseudoisochromatic plates are widely used; however, they have certain drawbacks which prevent an accurate accounting of a patient's deficiency. For example, the color hues on pseudoisochromatic plates are not calibrated. Thus, although the plates are very easy to use since the patient being observed only needs to look at the various different plates and inform the clinician when he sees a figure, the plates do not have precision in terms of determining the type or degree of color vision a person has. Further, pseudoisochromatic plates require that the patient be capable of pattern recognition; small children (or even adults) can have difficulty recognizing patterns; i.e., numerals or letters formed by dots. In addition, the plates tend to fade over time as they are handled. Any hand oils that are transferred to the cards alter the plate colors over time causing erratic readings.
Finally, the use of pseudoisochromatic plates requires special lighting. To work properly, the cards should be viewed under an illuminant C light (not a fluorescent or incandescent light). Problems arise in that many testers or testing centers have the cards but do not have the proper light; again, this results in inconsistent test results.
A second method commonly used and similar to the pseudoisochromatic plates is the use of slides containing colored dots. These slides work well in combination with acuity charts where a particular patient can have his distance and focus capabilities tested using a common testing apparatus. Slides have a lesser tendency to fade because they are normally handled less. However, slides still require special lighting; and faithful reproduction of the slides is difficult.
A third method of testing color vision deficiency is given in U.S. Pat. No. 3,947,099 to Grolman, et al. Here an anomaloscope that uses a bipartite field having a spectrally pure yellow half and a mixture red and green half for testing the color vision of a patient is disclosed. To perform an anomaloscope test, the patient is instructed to adjust two knobs until the bipartite field appears consistent in color. People who have red-green color deficiencies will either adjust the red-green ratio so that it is totally different from what a person with normal vision would perceive or they may have a lesser color deficiency and simply have a range in which the two halves of the bipartite field appear similar but are in fact different, as perceived by a person with normal color vision. This particular test requires substantial tester/patient interface to determine the range and degree of color deficiency that a patient has.
In addition, this test can be difficult for certain individuals, particularly small children. For instance, in order to adjust the spectral red and green colors of the mixture on one half of a display circle to correspond with the spectral pure yellow color on the other half of the display circle, the patient has to be able to adjust both the ratio of the red and green colors in the mixture and the brightness thereof, in order to look just like the spectral yellow color. There are versions of the anomaloscope that are computer controlled and require less interaction between the patient observer and the tester. However, such versions are expensive when compared to the use of pseudoisochromatic plates. Hence, because the anomaloscope can be difficult for a patient to operate, most clinicians, when testing color vision of a patient, resort to using pseudoisochromatic plates (an example of which is given in Hardy, et al. U.S. Pat. No. 2,937,567).
Another common method of testing for color vision defects, and one that is especially used in testing for blue-yellow color vision defects, is the arrangement test. In this type of test, the patient is presented with an array of colored chips embedded into caps, and is asked "to arrange the caps in order according to color." Misarrangements of the colored caps can be used to deduce color vision defects. Arrangement tests suffer from the same deficiencies as pseudoisochromatic plates, in that they require that the patient understand the task, the colors tend to fade over time as they are handled, hand oils transferred to the color chips will alter the colors over time, and special lighting is required. Additionally, the scoring and interpretation of the test is somewhat complicated.
A fourth method of testing color deficiency of a patient is given by Massof in U.S. Pat. No. 4,848,898. There, an instrument is disclosed which is similar to an anomaloscope but with a multiplicity of lamps. The instrument contains preset values consisting of mild, moderate or severe color deficiencies wherein a patient views a panel of lamps and is asked to pick out a particular light pattern which appears on the screen before him. Depending on the patient's ability to identify the pattern, color deficiencies can be determined within reasonable bounds.
With the Massof invention, it is often difficult for small children (and some adults) to evaluate specific patterns. Although they may recognize that certain lamps appear to be a different color, they cannot actually determine the pattern which exists.
With color vision testing it is important that the test be primarily focused on the fovea and that the field of testing not exceed 11/2-2 degrees of center. Large screen tests wherein a number of discriminating light sources are available to the patient, although he has a strong deficiency in the central focus area, i.e., the fovea, can result in some pattern recognition because of the large vision field while in fact the patient suffers some color deficiency. Also, the Massof instrument is limited to testing red-green color deficiencies.
Therefore, there exists a need for an instrument that can provide for both easy operation and classification of a patient's color vision defect.